Cable terminal



June 11, 1946. HE 2,401,996

CABLE TERMINAL Filed July 28, 1942 IIIIIIIIII- III/IIIIIIIIIII.

III/I Inventor": Z6 '7 Lynn Wetherill,

"I? by 15 His Attovneg.

Patented June 11, 1946 CABLE TERMINAL .Lynn Wetherill, Plttsfield,Mass., assignor to General Electric Com New York Da y, a corporation ofApplication July 28, 1942, Serial No. 452,573

10 Claims. 174-19) My invention relates to terminals for electric cablesand more particularly to a terminal for a I high voltage electric cableof the type utilizing a gas insulating medium under pressure.

The design of terminals for gas filled cable presents problems notencountered with terminals for use with ordinary solid cable. Generallyspeaking, it is advantageous to incorporate in the terminal the sametype of insulating medium asthat employed in the cable. This avoids thne'- cessity of relatively expensive barriers between the cable andterminal such as would be necessary if the insulating medium in'theterminal were different from that in the cable. In th case of cablehaving free feed channels therein containing insulating gas underpositive pressure, this involves the use of the insulating gas withinthe terminal. It is known, however, that a gas filling within a terminalhas an-appreciably lower dielectric strength than solid compound fillingordinarily used in such terminals. Nevertheless, for the reasonsindicated above it is desirable to use the gas insulating medium withinthe terminal. I have found it possible to do this by so designi theterminal construction that the electrical stresses are distributed overa region such that they are well below the breakdown point of the gasinsulating medium thereby preventing the formation of corona andprotecting the cable insulating material from'damage.

The object of my invention is the provision of a new and improvedterminal for a gas filled electric cable-s constructed and arranged asto prevent the development of electrical stresses capable of causingdielectric failure of the insulating medium incorporated into theterminal.

' In theaccompanying drawin i 1 is a view of my cable terminal with apart of the terminal broken away to show the interior construction;

Fig. 2 is an enlarged view of the cableand stress cone within theterminal. and Fig. 3 is a diagrammatic view of a gas filled cable suchas is used with my terminal.

2 formed of open spiral tubing. Since the shielding tapes I! aregrounded to the sheath bycontact therewith, the gas in the free feedchannels is also at ground potential. The gas insulating medium ispreferably nitrogen although other inert gases may be used; it issubjected to a relatively low pressure of approximately 15 to 30 poundsper square inch although higher pressure may be used as desired. Thecable thus brieflyv individual conductors separated for connection looseenough to provide a free feed gas channel,

ll between the sleeve and shielding, forming an extension of one of thegas channels II and through which gas may be fed into the terminal in amanner to be described later.

The terminal proper includes a casing [8 of cast metal supporting aninsulator IQ of conventional construction into which the cable isinserted. The insulation at the end of the cable is stripped back toexpose the conductor which is then fastened to a stud 20 carried at oneend of the insulator. At its opposite end the insulator is secured to aflange or sleeve 2| which serves to mount it on the casing. It isfastened to the sleeve by a metal to porcelain joint, shown at 22;asbestos packing}: cushions the seating of insulator on the flange 2!.This method of sealing the flange to the insulator forms no part of thepresent invention and may be done in any conventional manner dueprecaution being taken to make sure that the Joint is gas tight. Thesleeve 2|. in turn, is fastened to the casing II in any I suitablemanner as by a soldered joint 24. A suitable packing 25 of the asbestosor the like prevents any of the solder from entering the interior of thecasing. At the point where the lead sheath l6 enters the casing is acollar 26, preferably of lead, which serves a a base for the wiped 3lead joint 21 between the casing and the cable sheath. Here again, careshould be exercised to insure a gas tight Joint. The casing II andconsequently the entire terminal assembly are supported on a bracket 28extending laterally from the casing.

At a point adjacent that at which the lead sheath l6 terminates, theshielding tapes II are removed to expose the insulation 29 on theconductor for receiving a stress cone of insulating material. This pointis adjacent that at which the cable enters the casing and it is in thisregion that the electrical stresses between the shielding and theterminal are greatest, The stress cone includes a wrapping 30 ofinsulating material such as varnished cambric tapes which are appliedlayer by layer by wrapping directly around the factory insulation 2!. Aninsulating layer of appropriate thickness is thus built up along asubstantial length of the cable in the region of the base of theinsulator. Opposite ends of the layer are tapered, as indicated at II,to

form the beveled surfaces of the cone.

In order to distribute properl the electrical gradients within theterminal the insulating layer is provided with a shield which comprisesa copper braid 32 one end of which is connected to the cable shieldingtapes II and the other end of which extends up along the length of theinsulating layer 30. A layer of resistance material 38, for example, acarbon impregnated cloth. forms an extension of the copper shieldingbraid for a purpose to be described presently. The resistance layer isapplied in the form of a tape and extends for a short distance along thelength of the insulating layer 30, The copper braid is likewise appliedas a tape and overlies the resistance layer for a short distance to makegood electrical connection therewith, At the point where the coppershield overlies the cable shielding tapes II, it is soldered to them, asindicated at 34, t insure good electrical contact. Overlying theinsulating layer 30, the resistance layer I3 and a portion of theshielding braid 32 is a second layer 35 of insulating materialcomprising built-up wrappings of varnished cambric tape. The ends ofthis layer are beveled of! to form extensions of the beveled surfaces llof the underlying layer 30.

A barrier 36, in the form of a cylinder of insulating material, isplaced within the terminal as a further protection against corona. Thebarrier rests on the tapered inner wall of the casing II and extendsalong the length of the stress cone in spaced relation thereto. Thismeans that the gas within the casing and insulator may circulate freely.As already mentioned, gas may enter the terminal from the cable throughthe free feed channel I! between the cable sheath and shielding.However, I prefer to provide an additional source of gas supply directlyto the terminal since this is a convenient way to supply gas to thecable. To this end, I provide an inlet port ll formed in the casing llthrough which insulating gas under pressure may be fed directly into theterminal. The gas is maintained under the same pressure as in the cable,which may be approximately 15 to 30 pounds per square inch, althoughhigher pressure may be used.

The insulating gas within the terminal has inherently a lower dielectricstrength than the bituminous compound or oil ordinarily used in suchcases so that it has a tendency to break into corona at voltages whichwould be'withstood by the compound or oil. Moreover, it has a dielectricconstant, or specific inductive capacity, of approximately 1.0, ascompared with 4.0 or 5.0 for solid compound'or 2.0 to 5.0 for liquidcompound,

which means that the gas is actually subject to much higher electricalstress than the compound or oil would be. For these reasons the use of agas as an insulating medium in a cable terminal presents problems suchas are not encountered when liquid or solid fillers are used. However,my cable terminal is so constructed'as to provide for the use of such agas insulating medium and the design is such that the terminal isparticularly well adapted for use with gas tilled cable since the stressdistributing shield is protected from direct intimate contact with thegas insulating medium at the points of highest stress.

If the shield 32 were terminated abruptly on the outer surface of theinsulating layer II, as is commonly done today in the case of compoundfilled terminals, there would be danger that the gas filling would breakinto corona for the reasons already mentioned. I overcome thisdiilicuity by continuing the shield up into engagement with theresistance layer 33. This layer 83 of resistance or semi-conductinmaterial prevents the concentration of electrical stress and raises thevoltage at which corona will start. In order to prevent further theformation of corona streamers in the vicinity of the resistance layer, Iprovide the second layer 35 of varnished cambric tape which furtherraises the voltage at which corona will start. This is due not only tothe higher dielectric strength of the varnished cloth, as compared tothe gas filling, but also to its higher specific inductive capacitywhich reduces the stress it must carry. In general, the thickness of theinsulating layer outside the resistance tape and shield depends upon thevoltage at which the terminal is to be operated. The higher the voltage,the greater the thickness of the built-up layer required.

The resistance layer 33 serves the further purpose of decreasing theelectrical stress along the lengths of the insulating layers 30 and IIat a point adjacent the end of the copper shielding braid. Without theresistance or semi-conducting layer I3 the axial stresses along thesurfaces of the superposed layers of insulating tape build up to aconsiderable extent causing corona. However, the resistance layer servesto dissipate or distribute these stresses, thereby preventing failure inthat particular manner. The layers II and SI exclude the filler gas fromimmediate contact with the end of the shielding. except where the gasexists as thin films between ad- Jacent convolutions of the tape, sothat the resulting dielectric strength of the insulating mediumsurrounding the shielding is raised.

I prefer tomake the resistance or semi-conducting layer of a goodquality of cloth in which a suitable carbon bearing resin has beenimpregnated. It may have a surface resistivity oi 12,000 megohms persquare inch, that is, the resistance between opposite edges for a pieceof tape one inch square may be about 12,000 megohms. Although I preferto employ the cloth as a tape which may be assembled in position at thesame time that the insulating tapes of the stress cone are applied, itshould be manifest that the resistance layer may be constructed in otherways. For example, it may comprise a paper tape having a.semi-conducting painted surface or it may consist of a suitable plasticwith which carbon or other conducting material is mixed.

My terminal is simple in construction and is one which may bemanufactured at comparatively low cost. It is unnecessary to employ abarrier between the gas-filled cable and the terminal such as would berequired if a compoundfilled terminal were employed. Since the terminalis filled with the same insulating medium as that used in the cable, asingle source of'gas may be used to supply both elements.

What I claim as new and desire to secure by' Letters Patent of theUnited States is:

1. A terminal for a gas filled cable having a shielded insulatedconductor, an enclosing sheath and a gas feeding channel within thesheath comprising, in combination, a chambered means sealed at one endto the cable sheath, an insulator for the conductor supported by saidchambered means, a layer of insulating material surrounding theinsulation on the conductor, a layer of conducting material connected tothe conductor shield and extending over a portion of said insulatinglayer to form a shield, said conducting layer terminating in a belt ofresistance material, a second layer of insulating material overlyingsaid resistance material and a portion of said conducting layer, afilling of insulating gas within theinsulator and chambered means, and apassageway between said chambered means and the interior of the cablewhereby gas may flow freely between the cable and terminal.

2. A terminal for a gas filled cable having a factory shielded andinsulated conductor, an enclosing sheath and a gas feed channel withinthe sheath comprising, in combination, a casing sealed at one end to thecable sheath with the feed channel extending into said casing, a-hollowinsulator for receiving the conductor supported by said casing, acovering of wrapped-on insulating tape surrounding the factoryinsulation on the conductor, a cover of wrapped-on conducting tapeelectrically connected to the conductor shield and extending over aportion of said insulating covering, a covering of wrappedon resistancematerial electrically connected to said conducting material'and formingan extension thereof, a second covering of wrapped-on insulatingmaterial overlying said resistance materialand a portion of saidconducting cover, and a filling of insulating gas within the insulatorand easing.

3. A terminal for a gas filled cable having a factory insulated andshielded conductor, an enclosing sheath and a gas feeding channel withinthe sheath comprisinE, in combination, a casing sealed at one end to thecable sheath with the feed channel extending into'said casing, a hollowinsulator supported by said casing for receiving the conductor, afilling of insulating gas within the insulator and casing and a stresscone for the conductor comprising a covering of wrapped- 'on insulatingtape surrounding the factory interial and a portion of said conductingcovering.

4. A gas filled terminal for a conductor having factory appliedinsulation comprising a terminal 6 housing for the conductor, a fillingof insulating gas within said housing, and a stress cone for theconductor comprising a'built-up layer of insulating material extendingalong a short length of the conductor, a, layer of conducting materialoverlying a portion of the length of said insulating layer, a layer ofsemi-conducting material.

- length of the conductor and having a conical surface, a layer ofconducting material overlying said conical surface, a layer ofresistance material overlying a portion of said insulating covering andbeing electrically connected to said conducting material to form anextension thereof and a second covering of wrapped-on insulatingmaterial overlying said resistance material and a portion of saidconducting material.

6. A terminal for a gas filled cable having a factory insulatedconductor, an enclosing sheath and a gas feed channel within the sheathcomprising, in combination, a casing sealed at one end to the cablesheath with the feed channel opening into said casing, a hollowinsulator for receiving the conductor supported by said casing, a layerof insulating material surrounding the factory insulation on theconductor and extending along a short length of the conductor, a layerof conducting material extending over a portion of said insulatinglayer, a layer of semi-conducting material likewise overlying saidinsulating layer and being electrically connected to said conductingmaterial to form an extension thereof, a second layer of insulatingmaterial overlying said semi-conducting material whereby the latter isembedded between said layers oil-insulating material, and a filling ofinsulating gas within the insulator and casing.

7. A terminal for a gas filled cable having an insulated conductor, anenclosing sheath and a gas feed channel within the sheath comprising, incombination, a terminal housing for the conductor sealed at one end tothe cable sheath with the feed channel opening into said housing, astress cone for the conductor within the housing comprising a layer ofinsulating material surrounding the insulation on the conductor, a layerof conducting material extending over a portion within the housing.

8. A terminal for an electric cable having a conductor and an enclosingconducting covering comprising a terminal housing for the conductor,

means for connecting the housing to the conductlng covering, and astress cone for the conductor within the housing, said cone including aconical layer of conducting material with an end portion for electricalconnection to the conducting covering, and a layer of semi-conductingmateriai electrically connected to the other end portion of theconducting material of the conical layer to form an extension thereof.

k 9. A terminal for an electric cable having a conductor and anenclosing conducting covering comprising a terminal housing for theconductor, means for connectin the housing to the conducting covering,and a, stress cone for the conductor within the housing, said coneincluding a conical layer of conducting material with an end portion (orelectrical connection to the conducting covering, a layer ofsemi-conducting material electrically connected to the other end portionof the conducting material of the conical layer to form an extensionthereof, and a body of insulating material embedding the free end oi thesemi-conducting layer.

lO. The combination with an electric cable having an insulated conductorwith a shielding covering the insulation and an enclosing sheath, of ahousing for a portion 01 the conductor, the shielding being partlyremoved from said portion, means for sealing the housing to theenclosing sheath, and a stress cone for the conductor within the housingcomprising a body of insulating material wrapped around the conductorand having a conical portion terminating near the end of the shielding,and a, layer of conducting material covering said conical portion andhaving a. semiconducting portion extending along said body of insulatingmaterial, said layer being eletrically connected to the end 01 theshielding and having an electric resistatnce which is low near itsconnection to the shielding and high near its 20 free end.

